SEOUL - North Korea's army says it has approved plans for nuclear attacks on US targets, the latest in a series of apocalyptic threats that have sent tensions soaring on the divided Korean peninsula.
North Korea has moved a missile with "considerable range" to its east coast, South Korea's defence minister said, but he added that there are no signs that the North is preparing for a full-scale conflict.
The United States scrambled to reinforce its Pacific missile defences.
US Defense Secretary Chuck Hagel said Pyongyang's increasingly bellicose threats combined with its military capabilities represented a "real and clear danger" to the United States and its allies South Korea and Japan.
"They have nuclear capacity now, they have missile delivery capacity now," Hagel said Wednesday. "We take those threats seriously."
Shortly afterwards, the North Korean military said it had received final approval for military action against the United States, possibly involving nuclear weapons.
"The moment of explosion is approaching fast," the Korean People's Army general staff said, responding to what it called the provocative US use of nuclear-capable B-52 and B-2 stealth bombers in war games with South Korea.
The US aggression would be "smashed by... cutting-edge smaller, lighter and diversified nuclear strike means," it said in a statement.
The Pentagon said it would send ground-based THAAD missile-interceptor batteries to protect bases on Guam, a US territory some 3,380 kilometres southeast of North Korea and home to 6,000 American military personnel.
The US army withdrew all its tactical nuclear weapons from South Korea in 1991 and the South has no such weapons. But the US guarantees a "nuclear umbrella" in case its ally comes under atomic attack.
South Korean Defence Minister Kim Kwan-Jin said he did not know the reasons behind the North's missile movement, and that it "could be for testing or drills."
He dismissed reports in Japanese media that the missile could be a KN-08, which is believed to be a long-range missile that if operable could hit the United States.
Mr Kim told lawmakers at a parliamentary committee meeting that the missile has "considerable range" but not enough to hit the US mainland.
South Korea's main stock market registered its biggest one-day fall in four months on Thursday because of the escalating threats from North Korea, and fears about the future of the North-South joint industrial complex.
UN chief Ban Ki-moon said he was "deeply concerned" by rising tensions on the Korean peninsula after Pyongyang said it had approved plans for nuclear strikes on US targets.
Activist hackers - possibly the group known as Anonymous - appeared to have infiltrated North Korea's official news website Uriminzokkiri and its accompanying Twitter and Flickr feeds.
Mr Kim, the South Korean defence minister, said that if North Korea were preparing for a full-scale conflict, there would be signs such as the mobilisation of a number of units, including supply and rear troops. South Korean military officials have found no such preparations.
North Korea's recent threats "are rhetorical threats. I believe the odds of a full-scale provocation are small," he said.
But he added that North Korea might mount a small-scale provocation such as its 2010 shelling of a South Korean island, an attack that killed four people.
North Korea has been railing against joint US-South Korean military exercises that are taking place in South Korea and has expressed anger over tightened UN sanctions for its February nuclear test. Many of the threats come in the middle of the night in Asia - daytime for the US audience.
Analysts say the threats are probably efforts to provoke softer policies from South Korea, to win diplomatic talks with Washington and to solidify the image of young North Korean leader Kim Jong-Un.
At times, North Korea has gone beyond rhetoric.
On Tuesday, it announced it would restart a plutonium reactor it had shut down in 2007. A US research institute said Wednesday that satellite imagery shows that construction needed for the restart has already begun.
For a second day Thursday, North Korean border authorities denied entry to South Koreans who manage jointly run factories in the North Korean city of Kaesong. South Koreans already at the plant were being allowed to return home.